1. Field of the Invention
The present invention relates generally to an optical data recording medium. Particularly, the present invention relates to a readable/writable optical disk which is supported on a rotatable disk table when in use.
2. Description of the Related Art
In optical disk technology, a small laser spot is utilized for reading and/or writing high density information data to a recording playback layer of the disk material. Generally, three types of such disks are available, read only, write once read many and rewritable.
For many types of data which may now be accessed by computer users, such as coded data, audio data and graphic data, an erasable rewritable type of optical or magneto optical disk is the most useful.
Referring to FIGS. 5 and 6, a conventional example of such type of magneto optical disk will be discussed hereinbelow.
FIG. 6 shows a cross-sectional view of the conventional magneto-optical disk of FIG. 5 as supported on a disk table 6 of a disk drive (not shown) for carrying out reading/writing operations. The disk 1 comprises a disk substrate 2 both sides of which have an opening 2a formed at a center portion thereof. The disk substrate is formed of a combustible plastic resin generally molded by injection of the resin through the central openings. At one side of the disk substrate a circular plastic hub 3 having an installation groove 3a formed at an inner peripheral side thereof is provided.
Within the hub 3, retained by the installation groove 3a, a circular mounting plate 4 having a center opening 4a is provided. The center opening 4a allows insertion of an output shaft 5 of a spindle motor (not shown) of the disk drive (not shown) for rotatingly driving the magneto-optical disk 1.
The disk table 6 supporting the disk 1 is comprised of a table body 7 including an annular portion 7a and an annular disk support portion 7b of the table body 7, the disk support portion 7b being effective for determining correct positioning of the disk 1. The lower portion of the table body 7 is inset with a circular yoke 8 and, set between the yoke 8 and the circular mounting plate 4 an annular magnet 9 is disposed for effecting high speed rotation of the disk 1 according to operation of the spindle motor (not shown).
In addition, disposed around an outer periphery of the annular disk support portion 7b a friction ring 10, formed of rubber or the like, is provided for preventing slipping of the disk 1 when loaded on the disk table 6.
According to the above arrangement, when the disk substrate 2 is set on the table body 7 such that the circular mounting plate 4 is engaged by the magnetic force of the magnet 9 and the output shaft 5 of the spindle motor (not shown) is inserted into the centering opening 4a of the circular mounting plate, the outer circumference of the output shaft 5 is in touching contact with the inner cirumference of the centering opening 4a.
Such a conventional magneto-optical disk as described above has been disclosed, for example, in Japanese Utility Model 62-80240.
Increased demand for such variable magnetic overwriteable disk systems has been forecast, thus systems having advantages including low cost, simple single layer construction etc., are desired.
In such an optical disk system as described above, the disk substrate is generally formed of a polycarbonate resin, or the like, according to this, in order to minimize occurence of comma type aberration, a substrate thickness of 1.2 mm is set. At this, a flexibility or resilience of the substrate is lowered considerably. The effect of this is that when the hub 3 is chucked in the table body 7, the disk substrate 2 abuts the annular disk support portion 7b of of the table body 7 under pressure such that warpage of the disk substrate may easily occur.
A degree of warpage to which the substrate 2 is susceptible is directly connected to the resilience or flexibility of the substrate material.
At this, flexibility is proportional to thickness to the third power. That is, if theoretically, the warpage of a single layer substrate is expressed as S.sub.1 and of a two layer substrate as S.sub.2, comparatively; S.sub.1 =2.sup.3 .times.S.sub.2, i.e. S.sub.1 =8.times.S.sub.2.
Accordingly, such warpage, particularly in a single layer substrate type disk, cannot be ignored as this will have an adverse effect on data reading and writing operations and degrade disk performance.
Thus it has been required to provide an optical disk by which warpage of a substrate portion of the disk is prevented and degrading influence of warpage on data reading and writing operations is prevented.